This invention relates to pattern recognition techniques in general and, more particularly, to a method and apparatus for aligning a thin flat disc or square with respect to a repeatable pattern formed or implemented on the surface of the disc. In the preferred embodiment, the method and apparatus is employed to provide targetless alignment of a semiconductor wafer by utilizing the "street" pattern formed by the dies for identification.
In many semiconductor processing operations, the wafer must be accurately positioned so that the circuit patterns are aligned in X, Y and theta with respect to a predetermined object, such as, a photomask, prior photoresist patterns, probe tips, markers, bonders and the like.
Historically, the alignment function has been performed by human operators through microscope viewing and manual equipment manipulation. The manual technique suffers from disadvantages in handling damage, operator error and inconsistent performance.
In recent years, automatic alignment has been achieved in the photomask/wafer alignment field. Here, purposely designed target patterns were introduced in certain die positions of the wafer to facilitate automatic machine alignment. The so-called "target" technique has been implemented in volume production. One representative automatic aligner is the Cobilt AF-2800 automatic aligner sold by the Cobalt Division of The Computervision Corporation under the trademark AUTOLIGN.
Another alignment method that has been described in the literature utilizes the wafer circuit pattern in conjunction with an artificial boundary type target. The artificial target is superimposed on a TV camera image of the wafer. Representative examples of this technique are described in U.S. Pat. Nos. 3,814,845 and 3,899,634.
Both the special target type automatic aligners and the automatic aligners that use an artificial boundary type target are dependent on means for locating the circuit pattern substructure i.e., the alignment techniques are mainly die oriented as opposed to wafer oriented. In the case of the aligners that operate with special targets, the targets take up valuable wafer geometry and preclude aligning wafers without such targets. Although the artificial boundary type target aligners permit full utilization of wafer geometry, the boundary type aligners suffer from lack of global alignment ability and pattern dimension flexibility with respect to resolution.
It is accordingly a general object of the present invention to provide a method and apparatus for targetless alignment of a semiconductor wafer which eliminates the problems associated with the prior art aligners.
It is a specific object of the invention to provide a method and apparatus for aligning a semiconductor wafer which utilize the die pattern itself rather than any special targets.
It is another object of the invention to provide a method and apparatus for three axis alignment of a disoriented disc or wafer in a targetless mode.
It is still another object of the invention to provide targetless three axis alignment together with Z-axis compensation.
It is a further detailed object of the invention to provide a simple and relatively inexpensive method and apparatus for Z-axis compensation in semiconductor processing equipment, such as, probers.
It is a feature of the invention that the method and apparatus does not require any special targets on the semiconductor wafer and utilizes the die pattern itself for orientation.
It is another feature of the invention that the requisite degree of alignment accuracy is achieved through a readily implemented interactive process.